Ecological diversity of Polynucleobacter bacteria

The selection of ecologically coherent taxa is crucial for ecological investigation on macroorganisms, as well as on microorganisms. Ecologists investigating the ecology of bacteria face a big dilemma, because almost nothing is known about the ecological diversity within narrow phylogenetic groups of free-living bacteria. Furthermore, the majority of the bacterial diversity is still uncultured, and therefore not taxonomically processed. Due to these reasons, the selection of taxa appropriate for ecological research is very difficult for microbial ecologists. This project will investigate the ecological and genetic diversity within a narrow, monophyletic group of heterotrophic freshwater bacteria. The investigated group of bacteria is of great ecological significance in the pelagic zone of lakes, ponds, and large rivers. This group is present in a broad spectrum (pH, trophic status, etc.) of freshwater habitats, and comprises up to 60% of total bacterial numbers. The ecological diversity within this species-like group of bacteria will be investigated by characterization of the trophic niches of selected group members. Field studies focusing on populations inhabiting ecologically contrasting habitats will be combined with laboratory investigations comparing the ecophysiological adaptations of strains obtained from contrasting habitats.

Bacteria are the most abundant organisms in aquatic habitats like lakes, streams, and oceans, as well as in soil of terrestrial habitats. One liter of water from a naturally unpolluted lake contains, for instance, more than 1 billion (> 109 ) bacterial cells. Bacteria represent integral components of all ecosystems on earth, and perform ecological functions, which are essential for the various ecosystems. Only a very small fraction of the global bacterial diversity could be characterized and taxonomically described so far, thus, the majority of the natural bacterial diversity is currently represented by undescribed species. Many of the described species seem to have at least a widespread if not a cosmopolitan distribution. Furthermore, such species were observed in ecologically diverse habitats, which indicate broad ecological tolerances. Such distribution patterns and high ecological plasticity is exceptional among higher organisms. In consequence, one has to ask if such bacterial species represent ecologically and phylogenetically coherent groups, or if they consist of subgroups differing in ecological adaptations and occupying different ecological niches. We addressed this question by investigating a model group, i.e. Polynucleobacter necessarius, which is known to posses a cosmopolitan distribution and the ability to dwell in ecologically divers freshwater systems. A systematic survey on distribution and abundance of the taxon in stagnant freshwater systems of the Austrian revealed an ubiquitous distribution in habitats ranging from small temporary puddles on forestry roads to large, deep, oligotrophic lakes (e.g., Lake Attersee). The range of inhabited systems also included acidic bog lakes and alkaline high-mountain lakes. The development of a method for detection of phylogenetically defined subgroups of the species P. necessarius in environmental samples revealed differences in distribution between subgroups, which could be linked to environmental parameters. One subgroup was, for instance, exclusively detected in small acidic ponds located in mountainous area, while another subgroup was exclusively present in alkaline ponds and lakes. Extending the investigations to lakes in Antarctica (cooperation with Antonio Camacho, University of Valencia) resulted in indications of the presence of an endemic P. necessarius subgroup in Antarctica, and investigations on systems in the tropical climate zone (cooperations with several scientists) resulted in the indication of a restricted distribution of another subgroup to this climatically defined area. In addition, investigations on the ecological function of P. necessarius strains revealed pronounced differences at least for some of the subgroups. In conclusion, our investigations demonstrated that the investigated model species represents a phylogenetically coherent group but a ecologically highly divers taxon, which includes spatially and ecologically (ecological niche) completely separated subgroups. It has to be assumed that the observed intraspecific ecologically heterogeneity is not exceptional but is also present in other, probably in many bacterial species defined by the current prokaryotic species definition. Future ecological and taxonomic investigations have to consider this intraspecific heterogeneity. This heterogeneity is especially of relevance in investigations on the diversity of bacteria.